Zinc oxide varistors are employed in voltage surge arrester devices for shunting surge currents while maintaining the ability to operate under line voltage conditions. These varistors have a high exponent "n" in the voltage current relationship I=KV.sup.n for a varistor, where I is the current through the varistors, K is a constant and V is the voltage across the varistor. High exponent zinc oxide compound varistors can have sufficient resistance at normal line voltage to limit the current through the varistor to a low value, but resistance at high currents is such that the varistor voltage with surge current flowing is held to a level low enough to prevent damage to the insulation of the equipment being protected by the varistor.
Because the varistors are continuously connected from line-to-ground a continuous current flows through the varistor, and the current causes a small amount of power to be dissipated by the varistors at normal system voltage and a normal operating temperature. The magnitude of both the current and the resulting power increases as the varistor temperature increases. Some means must therefore be provided to remove heat from the varistor to prevent thermal runaway. The means must not only be capable of preventing thermal runaway under normal conditions, but it must also be capable of dissipating the heat resulting from high current surges. One effective means for removing heat from the varistor bodies employs an aluminum oxide filled silicone resin. Each individual varistor disc is cast within a thick quantity of the resin material prior to insertion within the surge arrester housing. The thick silicone material acts as a heat sink and eventually the heat is carried to the walls of the surge arrester body. The use of a silicone encapsulant for heat sinking zinc oxide varistors is described within U.S. Pat. Nos. 4,092,694 and 4,100,588.
Another method for cooling zinc oxide varistor discs is described within U.S. patent application Ser. No. 939,792, wherein zinc oxide varistor discs are fitted with a metal disc heat sink held in place by means of a flexible elastic sleeve. The metal disc-varistor combination is held in thermal contact within the surge arrester body by means of a resilient positioning member and axially applied spring force. The metal disc rapidly removes heat from the varistor body during surge conditions and transmits the heat to the heat radiating arrester housing through the flexible elastic sleeve surrounding both the varistor body and the metal disc. The thickness of metal discs required results in an arrester housing of significantly greater length. Controlling the length of the housing is an important consideration in surge arrester design because wind loading and earthquake resistance strongly depend upon housing length. In addition surge arrester cost and weight increase with arrester length. The purpose of this invention is to provide an efficient heat transfer assembly with surge arrester housings of reasonable length having superior heat transfer properties.